As various electronic apparatuses such as personal computers, digital cameras, cell phones, etc. have been tending to be smaller in size and higher in performance, there has been a rapidly increasing demand for semiconductor devices smaller in size, smaller in thickness and higher in density. In this connection, there is a keen expectation for development of a photosensitive insulating material which can cope with an increase in substrate area for attaining an enhanced productivity, can also cope with a structure having fine ruggedness with high aspect ratios on a substrate, in high-density mounting technology such as chip size packaging, chip scale packaging (CSP) or three-dimensional stacking, and is effective for use as an adhesive between stacked substrates.
As a photosensitive insulating material as above-mentioned, a photocurable resin composition has been proposed which can be applied in a wide range of film thickness by a spin coating method (commonly used in semiconductor device manufacturing process), can enable fine pattern formation using radiations in a wide wavelength range, and can give electrical/electronic part protective film excellent in flexibility, heat resistance, electrical characteristics, adhesion, reliability and chemical resistance through low-temperature post-curing (Patent Document 1: JP-A 2008-184571). However, when the photocurable resin composition is applied by spin coating to a substrate having a rugged substrate, it is difficult to coat the substrate substantially evenly. In such a case, therefore, gaps (voids) in the photocurable resin layer are liable to be generated at step portions on the substrate. Thus, further improvements in planarization and step coverage have been requested.
In addition, as the other applying method for substituting the above-mentioned spin coating method, a spraying method has been proposed (Patent Document 2: JP-A 2009-200315). In principle, however, this method is liable to result in generation of such defects as differences in height arising from ruggedness of the substrate, film interruption (film breakage) at pattern edges, and pinholes in bottom surfaces of recessed portions. In addition, where the substrate has large variations in height, the composition should have a high viscosity for obtaining a required film thickness, and there remain planarization and step coverage problems yet to be solved sufficiently.
Besides, there has been proposed a photocurable resin composition which includes a multifunctional epoxy resin and a phenolic curing agent and which enables pattern formation in high aspect ratios (Patent Document 3: JP-A 2007-522531). Dry films of 1 to 100 μm in thickness formed by using the composition and applications of these materials are also proposed in the Patent Document 3. However, the internal stress in the resin is high, and an improvement is needed as to flexibility in lamination (bonding) of substrates. In addition, since the resin is poor in transparency, light absorption would occur in the film coat, the sensitivity at the time of pattern formation of a thick film would be lowered, and an exposure energy difference would be generated between a resist surface layer and a bottom surface portion due to absorption by the film coat itself. This would lead to deterioration of shape (rectangularity), or to non-uniform sensitivity in the plane of the substrate having a considerably rugged surface. Furthermore, in relation to use for lamination (bonding) of substrates, there is a demand for formation of a film coat which is good in substrate adhesion properties and which is free from generation of degassing which, if generated, would cause lamination (bond) faults.